I'm writing this as someone who has mostly lived in the US and Canada. Personally, I find the whole "lying to children about Christmas" thing just a bit weird (no judgment on those who enjoy this aspect of the holiday). But because it's completely normalized in our culture, this is something many people have to deal with.
Two questions:
What age does this normally happen? I suppose you want the "magic of Christmas" at younger ages, but it gets embarrassing at a certain point.
And how does it normally happen? Let them find out from others through people at school? Tell them explicitly during a "talk"? Let them figure it out on their own?
There is no evidence that belief in Santa is harmful to children, nor is telling them the truth. They only believe in Santa for like maybe three years, and they’ll figure it out on their own. The vast majority of kids figure it out by age ~7-8. You can tell them whatever you want, it won’t matter either way.
If you do tell them the truth, or they figure it out on their own, be sure to also tell them that even if they don’t believe, other kids do, and being a Santa-truther will not win them any prizes or make them any friends. It’s a good lesson about living in a society.